Blooming vs. Anti-Blooming

An example of blooming
Some sensors, including the Kodak KAF series, offer an optional anti-blooming gate designed to bleed off overflow from a saturated pixel. Without this feature, a bright star which has saturated the pixels (much greater than 85,000 electrons) will cause a vertical streak. This can be irritating at best, and if the streak bleeds onto your target object, there is no way to recover the lost data. CCDs with Anti-blooming gate protection are NOT recommended for low light level work because of the reduced sensitivity of these devices IF a better option is available.

Anti-blooming gates built into the CCD occupy about 30% of the pixel area. The result is a 70% fill factor and reduced sensitivity and well depth. The reduced sensitivity means that you have to expose almost twice as long to get the same signal level as a CCD without the anti-blooming feature. Also, the area of the CCD occupied by the anti-blooming gate leaves a significant gap between pixels, reducing the effective resolution of the sensor. Most of the CCDs we sell, such as the Kodak KAF-1001e, KAF-4200, E2V 47-10 and 42-40 do not have anti-blooming gates. All of our Interline CCDs such as the kai-2020 and 4020 do have anti-blooming gates.

Below are illustrations of the differences between the types of CCDs. These vary, but illustrate the reasons for reduced well depth and sensitivity for CCDs with anti-blooming gate protection.

No Anti-Blooming Gate
Anti-Blooming Gate

Because of these drawbacks, users of CCDs without anti-blooming gates have chosen an alternate method to avoid blooming. Rather than taking a single long exposure in which blooming is almost certain to occur, take several short exposures, in which the brightest objects haven't begun to bloom, and stack the exposures together with image processing software. The signal-to-noise ratio remains the same as in the longer exposure, but the result is free of blooming.

Since opinions vary on which is the ideal method to achieve anti-blooming, we offer the following guidelines for selection:

  1. If non-blooming of bright objects is critical for your application, and if guiding twice as long to overcome the loss in sensitivity is not bothersome, then the anti-blooming option may be for you.

  2. For tri-color imaging, front-illuminated CCDs already have low response in the blue. Therefore, the lessened response with the anti-blooming gates will require extremely long exposures with the blue filter to obtain good color balance. For this and other applications that require good response to blue light, you may wish to use the stacking method to avoid blooming.

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